Mobile cellular communication networks employ multiple access schemes in which inter-symbol interference (ISI) needs to be combated through equalisation. Obvious embodiments are the TDMA (Time Division Multiple Access) based GSM (Global System for Mobile Telecommunications), now evolving into the enhanced data rates for GSM, GSM/EDGE Radio Access Network (GERAN), and the TD-CDMA (Time Division Code Division Multiple Access) based UTRA-TDD (UMTS Terrestrial Radio Access Time Division Duplex) network. The description below uses GSM terminology as an example, but the invention is not limited to GSM. The invention particularly relates to encoding and decoding of multi-layered signals transmitted over a Multiple-Input Multiple-Output (MIMO) frequency selective channel.
Spectrum has become a limited and expensive resource in mobile cellular radio communication networks. Therefore, much attention is given to improving the spectral efficiency. One method of increasing capacity without an increased bandwidth is to exploit multiple antennas at both transmitter and receiver. The channel between transmitter and receiver is a MIMO channel. Such a MIMO channel does offer a much greater channel capacity compared to a channel with one transmitting and one receiving antenna [1].
There exist several proposed techniques in which the MIMO channel is exploited to increase capacity. Some of the more attractive techniques for exploiting the MIMO channel are techniques in which data is divided into separate layers being transmitted simultaneously, and where each layer may in the receiver be demodulated and decoded separately from all other layers. A layered space-time architecture for multi-element antenna arrays proposed by G. J. Foschini [2] is now often referred to as BLAST (Bell-Labs Layered Space-Time Architecture), designed for systems with flat fading channels. The BLAST method can be divided into two sub-classes: Diagonal BLAST (D-BLAST) [3] and Vertical BLAST (V-BLAST) [4], which are shown in FIG. 1 for a transmission system with two transmit antennas. In another paper by Foschini et al, [5] it was mentioned that “with diagonal layering, some space-time is wasted at the start and end of each burst.” However this does not relate to avoiding problems with ISI when changing transmit antenna for the different layers. Instead they conclude that in the beginning and end of a radio burst there will be a decreased capacity with the coding algorithm and the receiver algorithm they apply.
In D-BLAST a stream of data is de-multiplexed into several sub-streams, or layers of data, each of which may be encoded and mapped onto symbols independently. At a given time each layer is transmitted by a separate antenna. In the transmitter the antenna to which a layer is coupled changes at regular intervals. A position in a burst, where a layer changes transmit antenna, will for simplicity be referred to as a border between two layers. The transmitting antenna of a layer is switched in a cyclic fashion so that each layer is in total transmitted an equal length of time from all antennas. The layers could switch antennas as slowly as is shown in FIG. 1, or as fast as every symbol. This serves to ensure that none of the layers experiences the worst transmission path for a complete burst. If one of the transmission paths is lost due to fading it could still be possible, thanks to the transmission from multiple antennas, to recover the layer through use of an error correcting channel code such as e.g. a convolutional code.
Also for V-BLAST a stream of data is de-multiplexed into several layers of data, each of which may be encoded and modulated independently. As opposed to D-BLAST, each layer is associated to one transmit antenna for the complete burst. This means that, if one transmit antenna is lost due to e.g. fading, a complete layer transmitted from that antenna will be lost.
In the receiver a staged demodulation and decoding is typically considered, where each layer is demodulated and decoded separately. In practice this requires multiple antennas also for the receiver. Typically the number of receive antennas should be at least as large as the number of transmit antennas. This allows the receiver to suppress all layers, except for the desired layer, from the received signals. After a layer has been demodulated and decoded it is cancelled from the received signal. Alternatively, a layer may be cancelled from the received signal directly after demodulation and before decoding. This may be preferable, either if a code with high code rate has been used, or if a layer has been encoded over several transmission bursts, in which case the receiver would need to receive all bursts, over which the layers have been encoded, before demodulation and decoding is to begin.
In a recent U.S. patent [6] a receiver algorithm is described where multiple receiver antennas are utilised to suppress co-channel interference (CCI) and then using a “Viterbi-equalizer” to take care of ISI. However this only relates to a specific receiver algorithm but not any method for the transmission.
For V-BLAST performance is improved if the receiver determines which layer had the best transmission quality, and then demodulates and decodes, alternatively only demodulates, that layer first.